WÜSTITE
Wüstite is a fairly rare oxide, found in meteorites and in some terrestrial geological situations.
It has no economic importance.
It is classified in the Periclase Group and has strong magnetism.
1. Characteristics
Crystal system: Cubic, hexaeoctahedral.
Color: Black or brown.
Habit: Crusts, intergranular fillings, massive formations, microspherulites, pseudomorphs.
Cleavage: No information available.
Tenacity: Brittle.
Twinning: No information available.
Fracture: No information available.
Mohs Hardness: 5 – 5.5
Parting: No information available.
Streak: No information available.
Lustre: Metallic
Diaphaneity: Opaque.
Density (g/cm³): 5.88
2. Geology and Deposits
Wüstite occurs in some meteorites and as microspherules, probably of extraterrestrial origin, in various geological environments.
It forms as an alteration product of other Fe minerals at high temperatures in highly reducing environments. In this way, it can be found in some anthropogenic scoria.
In natural geological situations, wüstite occurs only in special cases, including:
– some tuff-breccias, accompanied by magnetite, hematite, goethite, and native iron.
– basalts intruded into coals.
– inclusions in diamonds in kimberlites.
In addition, it precipitates from volcanic fumaroles on the ocean floor and is found in Fe-Mn nodules.
3. Mineral Associations
It is associated with native iron and iron (hydr)oxides such as hematite, magnetite, ilmenite, maghemite, lepidocrocite, and goethite.
Also with some sulfides such as troilite, pyrite, and pyrrhotite.
It is associated with the minerals of the native iron paragenesis: taenite (Fe,Ni), schreibersite ((Fe,Ni)3P), and kamacite (Fe,Ni).
In some cases, with gypsum, aragonite, akaganeite ((Fe3+,Ni2+)8(OH,O)16Cl1.25·nH2O), elyite (Pb4Cu(SO4)O2(OH)4·H2O), serpierite (Ca(Cu,Zn)4(SO4)2(OH)6 · 3H2O) and hercynite.
4. Transmitted Light Microscopy
This does not apply, as wüstite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: The hardness of wüstite upon polishing is equivalent to that of magnetite, and its polished finish is of excellent quality.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium gray with a greenish tint.
Compared to hematite, the color is darker.
Compared to goethite, the color is lighter.
Compared to magnetite, the color is slightly darker.
Pleochroism: No.
Reflectivity: 18.65%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Wüstite is an isotropic substance.
Internal reflections: No.
May be confused with: many other minerals, it is necessary to consider paragenesis of native iron. None of the minerals listed below show bireflectance neither internal reflections. Pleochroism is absent or very faint.
Native iron has pure white with a bluish tint reflection color and reflectivity of 58%.
Cohenite has white cream reflection color and reflectivity of 57-59%. Weak anisotropy is diagnostic.
Schreibersite has light cream reflection color and reflectivity of 52%. Anisotropy between gray and brown.
Taenite has dark gray reflection color (like silicates) and reflectivity of 7% (very dark). It is isotropic.
Troilite has light brown reflection color (like pyrrhotite) and reflectivity of 40%. Distinct anisotropy between gray and bluish.
General Characteristics:
Replacement of wüstite by magnetite occurs, and wüstite may be present only as replacement relics. Wüstite is also replaced by hematite (specularite) and goethite.
Alteration of wüstite leads to the formation of magnetite with native iron.
Inclusions, very small, of native iron may occur in wüstite.
Zoning may be very well developed due to variable MgO contents.
